GPU rendering out of memory
As first I must say only for correctness: Does not matter, how much memory have you graphics card. Important is how much free memory have your graphics card, when you start render. Keep in mind: system need some video memory for running. 

You are not only one who have this problem. I thing Blender have some issues with freeing VRAM. I used Win and now I'm using Linux and it's the same. What help is restart Blender.

About peak of used VRAM - I don't thing this is correct number in case of error. From my experiences, if Blender give you "out of memory" error, this number is incorrect. You can determine exact size of scene by switch to CPU rendering and you can see how much memory is really used for your scene.